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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/108953] inefficient codegen for trivial
 equality
Date: Mon, 27 Feb 2023 17:11:47 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108953

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c++                         |tree-optimization
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I don't think there is anything the C++ FE should do here, user can write it
like that
by hand too:
bool check(C const& lhs, C const& rhs) {
  if (lhs.a !=3D rhs.a) return false;
  if (lhs.b !=3D rhs.b) return false;
  if (lhs.c !=3D rhs.c) return false;
  if (lhs.d !=3D rhs.d) return false;
  if (lhs.e !=3D rhs.e) return false;
  if (lhs.f !=3D rhs.f) return false;
  if (lhs.g !=3D rhs.g) return false;
  return true;
}
etc., so we should pattern match this somewhere during GIMPLE opts.

The memcmp issue is a RTL issue (so I think we want actually two bugs), at
GIMPLE level it is optimized into:
  _1 =3D __builtin_memcmp_eq (lhs_3(D), rhs_4(D), 12);
  _5 =3D _1 =3D=3D 0;
  return _5;
which is I think what we want.

The reason for doing conditional branch after each comparison is
targetm.compare_by_pieces_branch_ratio (DImode) tells it not to batch more =
than
one.
Not really sure if we should override it to something else and on which
targets.

The reason for the weird
        mov     eax, 1
        test    eax, eax
        sete    al
instead of
        xor     eax, eax
and
        xor     eax, eax
        test    eax, eax
        sete    al
instead of
        mov     eax, 1
is that the epilogue is threaded only too late where the constant propagati=
on
through it isn't possible.=